It is known that nitrate nitrogen present in food is partly reduced into nitrous acid by enteric bacteria in the living body and the nitrous acid thus formed results in the formation of mutagenic nitrosamine through its binding with secondary amine present in other food. Nitrosamine is toxic for human health because it is considered to trigger the formation of cancers, tumors and the like. In consequence, great concern has been directed toward the reduction of nitrate nitrogen content in food.
In addition, since the incorporation of nitrate nitrogen into the human body originates from food, particularly from vegetables, it is most important to reduce the nitrate nitrogen content in vegetables.
On the other hand, the problems caused by nitrate nitrogen are common not only in human provisions but also in forage crops for domestic animals. Because of this, a standard nitrate nitrogen content in feed has been established, and reduction of the nitrate nitrogen level in feed therefore is an essential subject to be accomplished.
In view of the above, cultivation methods, especially fertilizer application methods, have been examined for the reduction of nitrate nitrogen content in plants. In these methods, the amount of fertilizer applied is reduced or a slow-acting fertilizer is used. However, such countermeasures limit the quantity and quality of fertilizers and therefore cause problems of slow growth and small yield of plants in comparison with commonly used cultivation methods.
Also, a method in which the nitrate nitrogen content in plants is reduced by the use of a fertilizer containing a nitrification inhibiting agent has been developed and partly put into practical use, but such a method resulted in high cost and its effect in reducing the nitrate nitrogen content was not sufficient.
Oxalic acid is an organic acid widely distributed in plants and takes part in the cause of the harsh taste of plants as a component of the "lye". In consequence, the oxalic acid content is one of the important factors which determine the quality of vegetables and the like. Especially, with the recently increasing occasion of eating vegetables raw in the form of salad and the like, great concern has been directed toward the reduction of the oxalic acid content in vegetables from the standpoint of taste.
On the other hand, when vegetables are considered from the dietetics point of view, oxalic acid is not desirable because of its nature to inhibit absorption of calcium into the living body by forming an insoluble material through its bonding with calcium. Also, oxalic acid causes calculus because it occupies more than 60% by weight of the urinary stone components.
In addition, since incorporation of oxalic acid into human body originates mostly from vegetables, it is important to reduce the oxalic acid content in vegetables.
In the conventional cooking of spinach, komatsuna (a kind of Chinese cabbage) and the like, their oxalic acid content is reduced by the so-called "removal of harsh taste" in which water is allowed to slop over in boiling them, but merely about 20% of the total amount of oxalic acid is reduced by such a means.
Though attempts have been made to produce vegetables with low oxalic acid content by selecting appropriate varieties and controlling fertilizer application, harvesting period and the like, satisfactory results have not been obtained yet. Also, very little actually is known about success in the development of a chemical agent which can reduce the oxalic acid content in vegetables.